United States

Abstract Body: AlInN has gathered attention as a candidate material for power electronics and optoelectronics. Historically, there were many challenges in growing AlInN by traditional MBE or MOVCD due to conflicting issues of a) phase separation for high temperature growth, and b) low adatom surface mobility for low temperature growth. Metal Modulated Epitaxy (MME) has proven useful for this challenging material by improving adatom kinetics at low temperature and was shown to improve X-Ray diffraction figures of merit by as much as 11X1. However, the source of the film improvement has been unknown up to now. TEM is a useful tool to investigate defect structures but can itself cause damage and induce phase separation if not performed under proper conditions. Of the few TEM reports of AlInN, most focus attention to AlInN around 18% Al and electron-beam induced degradation on AlInN is not investigated. Few reports exist for imaging high In AlInN. The present work focuses on the investigation of MME grown high In AlInN/GaN heterojunctions, explanation of MME’s improved quality compared to prior literature and its potential false defect observation because of damage induced by the electron beam in the TEM. Unintentionally doped (UID) MME AlInN films were grown on MME GaN on hydride vapor phase epitaxy (HVPE) GaN templates on c-plane sapphire substrates as described elsewhere1, using plasma-assisted molecular-beam-epitaxy (PA-MBE) with a target composition of 70% indium. Strain relaxation at a heterojunction of AlInN/GaN in this epitaxial film is investigated by TEM and Scanning TEM (STEM). The cross-section sample is prepared by standard mechanical polishing methods, followed by argon ion milling. Lattice-expansion mapping (or strain mapping) obtained by STEM nano-beam diffractions showed an intermediate lattice constant between AlInN and GaN. This implies that there is a thin intermediate layer where gradual strain relaxation happens from GaN towards AlInN. Moire fringes are detected in the TEM with two-beam condition of g = [1 -1 0 0], indicating full strain relaxation beyond ~10 nanometers via the generation of misfit dislocations at the interface. Moire fringes in MME grown InGaN/GaN resulted from strain exceeding the critical thickness in the first monolayer2,3. Thus, like MME grown InGaN which also has high adatom mobility at low temperatures, high In AlInN appears to relax via the same misfit dislocation mechanism resulting in higher film quality above the misfit array. In order to validate the results, it was also necessary to ensure the electron beam did not damage the films under examination. It was found that a low beam dose of 5.7 A/cm2 was necessary to prevent damage in thicker regions of the AlInN but still appreciably damaged thin sections of the AlInN even for less than 20 seconds exposure. This beam damage threshold is far lower than the ~35 A/cm2 work on InGaN/GaN4 suggesting high In AlInN is more sensitive to beam damage than InGaN. In-situ TEM measurements on g = [0 0 0 2] detected decomposition and In clustering in/on the AlInN layer. The beam-induced damage was not observable in the low-resolution strain map obtained by STEM micro-beam diffraction in comparably thicker regions. This extremely low beam damage threshold complicates TEM analysis of high In AlInN and results in less clear images and quicker analysis time requirements. Further work quantifying the degradation from the electron beam as well as varied compositional studies are ongoing. 1Engel, et. al., J. Appl. Phys., 127, 125301, 2020 2Fischer, et. al., Appl. Phys. Lett., 103, 131101, 2013 3Clinton, et. al,. Solid-State Electronics, vol.136, pp. 3, 2017 4Smeeton, et. al., Appl. Phys. Lett., 83, 5419, 2003

EMC 2021

Observation of Interfacial Strain Relaxation in High Indium, AlInN/GaN Heterostructures by Transmission Electron Microscope

alinn

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. This year’s event will feature a plenary session, topical sessions, a poster session and more.

EMC is coordinated with the Device Research Conference (DRC), which will be held during the same week, from June 20-23. This recognizes the strong interaction between device and electronic materials research and provides fruitful exchange of information between attendees of both Conferences.

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technical paper

Abstract Body: Controllable n-type doping in AlGaN and AlN is crucial for applications in electronics, optoelectronics, and power electronics. Si and recently Ge have been utilized as n-type dopants in AlxGa1-xN with low ionization energy for x<0.8 and x<0.5, respectively.1,2 However, the transition of the Si and Ge shallow donors to deep states occurs at x>0.8 and x>0.5, presenting challenges for novel applications employing Al rich AlGaN and AlN. While such a transition was previously observed in the AlGaAs system, it was confirmed to be an acceptor type DX-1. However, no such direct proof of the Si and Ge charge states in Al rich AlGaN and AlN exists. In this work we present direct evidence of single electron occupancy in the Ge deep state in AlxGa1-xN (x>0.5) i.e., Ge transitions to a deep neutral donor. Co-doping with the shallow donor Si (at x < 0.8) was used as a source of free electrons in Ge doped AlGaN grown via MOCVD to follow on the charge state of Ge. The measured carrier concentration (n) in Al rich AlGaN shows whether Ge is a shallow donor (n~[Si]+[Ge]), neutral deep donor (n~[Si]) or a compensating acceptor type DX-1 (n~[Si]-[Ge])). Temperature dependent carrier concentration measurements suggested an increase in the ionization energy of Ge: AlGaN with increase in Al content and a sudden drop in the carrier concentration was shown to occur at x>0.5 showing that Ge transitions to a deep state.1 Si remained a shallow donor with low ionization energy up to x~0.8. Therefore, both dopants are expected to be shallow donors in Al0.4Ga0.6N forming a d+ state. However, possible electron compensation in Si and Ge codoped Al0.65Ga0.35N where Ge transitions into a deep state, would directly show the charge sate of the deep state Ge, either D0 or DX-1. Carrier concentration measurements versus Al content under constant Si and Ge concentrations of 1×1019 cm-3 and 8×1018 cm-3, respectively, were realized at RT and 600°C. Negligible compensation and a temperature independent carrier concentration are observed in Al0.4Ga0.6N were both Ge and Si are expected to be in their shallow states. Interestingly, a reduction in carrier concentration at x~0.65 corresponding only to the Ge concentration was observed, indicating that Ge is not providing free electrons nor compensating the free electrons provided by Si. This is the signature of a deep donor having a higher ionization energy and not of an acceptor state. At higher temperatures, the ionization of the deep donor Ge provides higher carrier concentrations as expected for such a donor. Contrary to the formation of a stable DX-1, which is expected to pin the Fermi level at the (-/+) thermodynamic transition3, transition to a deep donor allows for the possibility of highly conducting AlN and Al rich AlGaN by degenerately doping the system. Hence, this work cast light on the true nature of donor related deep states in nitrides, further opening pathways for doping engineering and achievement of high conductivity in Al rich AlGaN and AlN. 1. Bagheri, P. et al. The nature of the DX state in Ge-doped AlGaN. Appl. Phys. Lett. 116, 222102 (2020). 2. Collazo, R. et al. Progress on n-type doping of AlGaN alloys on AlN single crystal substrates for UV optoelectronic applications. Phys. Status Solidi C 8, 2031–2033 (2011). 3. Gordon, L., Lyons, J. L., Janotti, A. & Van de Walle, C. G. Hybrid functional calculations of D X centers in AlN and GaN. Phys. Rev. B 89, 085204 (2014).

Direct Evidence of Ge Becoming a Deep Donor Instead of a DX- in Al Rich AlGaN

Abstract Body: Control over dopant concentration and distribution is essential to engineering nanowire-based devices. However, controllable n-type doping has remained challenging for GaAs, which is the foundation for many important III-V heterostructures. In particular, silicon behaves as a p-type dopant in GaAs nanowires grown by a vapor-liquid-solid (VLS) process in molecular beam epitaxy (MBE) conditions. Recently, it was shown that Si can act as a donor when incorporated in a catalyst free vapor-solid (VS) growth process,[1] highlighting the importance of understanding the growth and dopant incorporation mechanisms to controlling conductivity and carrier type. Here we present evidence that Si stabilizes the nanowire facets necessary for nanowire elongation, leading to uniform doping in a core region and enhanced doping at the nanowire surface. The growth conditions used here produce crystalline clusters of GaAs on Si (111) substrates in the absence of a Si flux. The introduction of Si leads to a high yield of uniform vertical nanowires (Figure 1(b)). We used atom probe tomography (APT) to measure the Si distribution in three dimensions and resolve variations in doping across the diameter of the nanowire. The reconstructions in Figure 2(a) and Figure 3(a) show a portion of the GaAs nanowire and the AlGaAs passivation layer (the entire diameter is not within the field of view). The dopant concentration increases from the nanowire center to the edge and peaks near the interface between GaAs core and AlGaAs shell (Figure 2(b)), indicating that different incorporation rates may be associated with distinct facets whose prevalence is influenced by the Si dopant itself. Additionally, Si-enriched regions and clusters appear at the intersection of {110} facets of some nanowires (Figure 3(b)), which maybe be linked to variations in surface faceting. The above observations suggest that Si alters the surface energy of GaAs and stabilizes the facets that promote nanowire growth via preferential incorporation. Si has previously been observed to induce twin-defect mediated growth in GaAs,[2] which may also be related to modification of surface energies. Surface etching to remove Si enriched layers provides a pathway to uniformly doped n-type GaAs nanowires that are free of catalyst impurities. References: [1] Ruhstorfer, D., et al. Demonstration of n-type behavior in catalyst-free Si-doped GaAs nanowires grown by molecular beam epitaxy. Applied Physics Letters, 2020, 116, 052101. [2] Keitaro Ikejiri, et al, Growth characteristics of GaAs nanowires obtained by selective area metal-organic vapor-phase epitaxy. Nanotechnology, 2008, 19, 265604.

Si Dopants Enable Catalyst-Free n-Type GaAs Nanowire Growth by Modifying Facet Energies

Abstract Body: We have used depth-resolved cathodoluminescence spectroscopy (DRCLS) to measure defects, defect complexes, and dopant densities localized at nanoscale interfaces of ultrathin indium tin oxide (ITO) on fused silica. Electronic and geometric structural properties of ITO in ultrathin, degenerate thin films strongly affect its transport properties on a nanometer scale. ITO grown by pulsed laser deposition (PLD) is a degenerately doped semiconductor due to Sn cation dopants supplied by Sn alloying into the otherwise In2O3 lattice. With its 3.5 eV or higher band gap and high conductivity, ITO is widely used as a transparent conducting oxide (TCO). Degenerate charge densities are achievable even with ultrathin films of 10 – 20 nm thickness and can provide plasmonic coupling with light via a tunable refractive index. Indeed, such nanoscale film thicknesses maximize electric fields in electric double layers used to modulate amplitudes of photons propagating in, i.e., optical waveguides to maximize tunability. Using ITO for electro-optic modulators enables devices to be made smaller and operate at higher frequencies than conventional materials1, but the minimum electrical or effective thickness of such highly conductive dielectric layers is less than the actual material thickness due to a disordered layer.2 The nature of such disordered layers in ITO and other TCO semiconductors has not yet been explored at the nm scale of films and can involve a combination of native point defects, lattice disorder, film morphology, and ITO/substrate interface chemical structure. Our DRCLS results for ITO films grown by pulsed laser deposition reveal the presence of new defects localized within 1 -3 nm of the ITO/glass interface that can account for lattice disorder and disordered transport layer. This is in agreement with quantum magnetoconductivity measurements, which find strong disorder at the interface that falls off exponentially toward the surface with a decay constant of about 6 nm. Figure 1 illustrates Monte Carlo rates of electron-hole pair creation inside In2O3:10%SnO2 films on fused silica as a function of incident electron beam energy EB showing that 0.4 kV to 1.5 kV EB controls measurement depth on a near-nanometer scale. Figure 2 illustrates significant changes in CL spectrum shape as a function of depth in the 13.1 nm thick ITO film, arising due to variations in defect density and band gap intensity. Defects are labeled with their tentative identifications. Figure 4 shows the CL emission spectrum at the interface with both ITO and SiO2 characteristic spectra subtracted out, leaving only features unique to the interface. It reveals a prominent feature at the interface at 3.38 eV and several smaller features at sub-EG energies. Figure 3 illustrates the subtraction process and Fig. 4 shows the deconvolved spectra used to generate normalized depth profiles. Figure 5 displays emission area as a fraction of total spectrum emission as a function of depth in the 13.1 nm thick ITO film, showing peak intensities in several defect features at or near the interface. Figure 6 depicts one possible band diagram of ITO that could produce luminescence features like those seen in our ITO CL spectra. Other possible explanations for various features include H complexes with native point defects, H interstitials, Sn-related traps, or In interstitials paired with two indium vacancies.[3] Defect and doping densities show significant differences between bulk-like films of several hundred nm versus ultrathin films in the 5 – 40 nm range, suggesting new approaches to minimize disordered layers and optimize interface conductivity. LJB and JC gratefully acknowledge National Science Foundation Grant no. DMR-18-00130. 1. K. Shi and Z. Lu, J. Nanophotonics 9, 093793 (2015). 2. D.C. Look and K.D. Leedy, Sci. Repts. 9, 1290 (2019). 3. J.B. Varley, H. Peelaers, A. Janotti, and C.J. Van de Walle, J. Phys.: Condens. Matter 23, 334212 (2011).

Defects and Dopants at Ultra-Thin Film Indium Tin Oxide (ITO) Interfaces

Abstract Body: Transparent oxide semiconductors (TOSs) that transmit deep ultraviolet (DUV, 200−300 nm in wavelength) are considered to be preferable as the active material for next generation optoelectronics such as biosensor.[1,2] Among many TOSs, La-doped ASnO3 (A = Ba, Sr and Ca) are considered as a promising candidate f DUV-TOS because of their rather high electrical conductivity and large bandgap (Eg). The optical Eg of electron-doped BaSnO3 and SrSnO3 are ~3.1 eV[3] and ~4.6 eV[4], respectively. Based on this trend, if Ca, an even smaller alkaline element, occupies the A-site, further widening of the Eg value can be expected. However, the effect of A-site substitution on the optical and electrical properties of ASnO3 has not been clarified in detail due to the lack of a systematic study. Here we show that the optical and electrical properties of ASnO3 films can be tuned systematically by changing the average size of A-site ions.[5] We used lattice parameter as a bridge to build the relationship between the A-site substitution and structure factors, and clarified the effect of lattice parameter on the optical and electrical properties. The lattice parameter almost linearly increased from 3.95 to 4.14 Å with increasing A-site ionic radius from 1.34 Å (Ca2+) to 1.61 Å (Ba2+). The optical bandgap (Eg opt) of the resultant films gradually decreases from ~4.6 to ~3.6 eV with increasing lattice parameter with a small positive bowing, while the electrical conductivity gradually increased from ~100 to ~103 S cm−1 due to gradual increases in both the carrier concentration and mobility. These results indicate that larger A-site ion substitutions reduce the Eg opt and the Fermi energy exceeds the mobility edge (degenerate semiconductor). On the other hand, the carrier generation efficiency increases when the Eg opt decreases. The carrier relaxation time increases when the mobility edge decreases. Thus, the conductivity increases when larger ion is substituted in the A-site. Meanwhile, La-doped SrSnO3 exhibited a great balance of wide bandgap and high electrical conductivity, which makes it a suitable ASnO3 system for advanced optoelectronic applications. The present results are essentially important for designing ASnO3 based DUV-TFTs. References [1] M. Wei et al., Adv. Electron. Mater. 6, 2000100 (2020). [2] M. Wei et al., Appl. Phys. Lett. 116, 022103 (2020). [3] H.J. Kim et al., Phys. Rev. B 86, 165205 (2012). [4] Q. Liu et al., J. Alloys Compd. 717, 62 (2017). [5] M. Wei et al., ACS Appl. Electron. Mater. 2, 3971 (2020).

Modulation of Optical and Electronical Properties for Transparent Oxide Semiconductor ASnO3 by the A-Site Ion Substitution

Abstract Body: Amorphous oxide semiconductor (AOS) thin film transistors (TFTs) based on In2O3 have attracted much interest for use as pixel switching elements in next generation active-matrix liquid crystal and active-matrix organic light emitting diode displays. The high field effect mobility of In2O3-based AOS devices (10–25 cm2/Vsec) offers significant performance improvements over present-day a-Si TFTs (<1 cm2/Vsec) technology. There have been considerable efforts to incorporate AOSs into devices, particularly in current-driven active matrix displays such as organic light emitting diode displays as pixel-driving switching elements. Strategies to enhance amorphous phase stability, improve channel/metallization contacts, and suppress channel carrier densities have been studied and successfully applied to the switching TFT application. Third cation elements are often added to binary cation oxide systems to limit the channel carrier generation for TFT channel application. We have recently reported that the addition of Al to InZnO, a typical binary cation material system leads to enhanced amorphous phase stability, carrier suppression capability and higher carrier mobility, up to ~45 cm2/Vs (Hall Effect mobility) and ~20 cm2/Vs (field effect mobility)1. All of these characteristics are expected to be a key enabler for realizing the next generation ultra-high-definition displays. Post-process annealing is widely employed in AOS-based TFT fabrication since annealing has been shown to improve field effect mobility2-3 and channel/metallization contacts4-5 as well as reduce trap density6-7. However, the post-annealing is often accompanied by an increase in channel carrier density that induces an unfavorable increase in the device off-state current and operation voltages2, 8. To date, the origin of the increase in channel carrier density has not been fully understood. Furthermore, it has been reported that gate-bias stress (either positive or negative) leads to TFT threshold voltage (VTh) instabilities. In order to achieve reliable device performance by mitigating the VTh shift issues, the understanding of the mechanisms for the VTh instability is to be enhanced. The current study aims to identify the origin of an increase in carrier density after low-temperature annealing conducted in air, particularly for a third-cation AOS system of InAlZnO (IAZO). Through work function investigations and bandgap analysis, the carrier density of IAZO is found to be increased by > 1E4 times compared to that of unannealed IAZO after low temperature annealing at 200 °C in air. Photoelectron spectroscopic studies reveal that the typical intrinsic (vacancy-based native defect) or extrinsic (cation substitution) doping mechanisms are not the primary cause of the channel carrier increase. From high pressure oxidation with much enhanced reactivity of reaction gases, it is identified that the equilibrium carrier density of IAZO is much higher than those used in typical TFT channel application. The low channel carrier density tends to increase and reach the higher equilibrium carrier density in the absence of kinetic constraints. In addition, to investigate the bias stability of IAZO TFTs, the transfer characteristics were measured over bias stress time at a negative gate bias of -30 V and a positive gate bias of 30 V. During the negative bias stress (NBS) tests, no significant changes were observed in transfer characteristics, which is attributed to the depletion of channel free carriers due to the application of negative bias, regardless of stress time. However, clear positive shifts of the TFT transfer characteristics are detected during the positive bias stress (PBS) measurements as shown in the figure above. TFT parameters such as field effect mobility, sub-threshold swing, VTh and interfacial trap density were carefully analyzed during PBS measurements. (Financial support: NSF Award No. ECCS-1931088)

Thermal and Bias Stability of InAlZnO-Based Amorphous Thin-Film Transistors

Abstract Body:Herein we p resent details of the photoluminescence, photoconduction and polaronic charge transport studies on crystalline quasi-2D γ-alumina synthesized by the two-step graphene assisted Atomic Layer Deposition (ALD) [1]. The Crystallographic and morphology analysis reported prior by us reveals the synthesized 2D nanosheet network to be polycrystalline cubic spinel γ-alumina with a close packed cubic texture in the <111> ALD induced growth direction whereas the morphology of 2D flakes was probed by both SEM and TEM and results point to the nanosheets with nominal thickness in the order of 10nm being sintered together achieving a large thickness for the 2D flakes. Photoluminescence (PL) spectroscopy of crystalline quasi-2D γ-alumina was done by exciting the network with ~320 nm (3.8 eV) light and the samples possess a single emission band centered at ~387 nm (3.2 eV) at the room temperature. The PL-excitation (PLE) measurements confirm presence of three excitation defect bands located within the band gap of 2D γ-Alumina. The PL band at 387 nm (3.2 eV) is assigned to non-interband, intrinsic defect-assisted radiative recombinations involving F, F2+ and Ali+ defect states. [2] Charge transport mechanism of the 2D-γ-alumina network was investigated by carrying out temperature- and wavelength dependent photoconduction measurements and the charge transport is confirmed to be polaronic with small polarons undergoing a field-induced drift within the polycrystalline network which is in part facilitated by thermally-assisted hopping as well as quantum mechanical tunneling. The polaronic charge transport shows two separate regimes characterized by two activation energies with the polarons bound to deep trap states controlling the charge transport at T>303 K. The RT conductivity of the network exceeds by up to four orders of magnitude that of prior reported by Morell for ~99.5% pure α-alumina.[3] Bandgap energy reduction, presence of transport-active defect centers [4] and texture [5] are likely to play a key role behind enhanced electrical conductivity of 2D-γ-alumina. Intensity dependent photocurrent measurements show that when samples are excited with photons with energy of 4.1 eV (300 nm) the current drops, and the change in the photocurrent depends linearly on the excitation intensity. These results indicate that crystalline, quasi-2D γ-alumina synthesized by the facile graphene assisted ALD synthesis could be a candidate for multiple device applications ranging from photodetectors, photocatalysts to radiation-hard, thermally-stable light emitters, and related nano-optoelectronic devices. Ref. [1] E. Kheirandish, M. Schofield, M. Gajdardziska-Josifovska, N. Kouklin, Advanced Material Interfaces, 7, 2000561 (2020)] [2] E. Kheirandish, N. Kouklin, Journal of Electronic Materials Topical Collections 62ndElectronic Materials Conference (2021) [3] R. Morell, Handbook properties of Technical Engineering Ceramics, Part 2, Data Reviews, Section I, High Alumina Ceramics, London, p.255, 1987 [4] B. Ealet, M. Elyakhloufi, E. Gillet, M. Ricci, Thin Solid Films, 250, 90-100 (1994) [5] A.Tan, C.K. Kuo and P.S. Nicholson, Solid State Ionics, 42, 233-241 (1990)

Charge Transport and Photoconduction Effects in Quasi Two-Dimensional γ-Al2O3 Grown by Graphene Assisted Atomic Layer Deposition

Abstract Body: Lead chalcogenide (PbX, X = S, Se, Te) colloidal quantum dots (CQDs) support rich optoelectronic phenomena, offering an attractive platform for advanced optoelectronics based on low-dimensional materials. Experimental results have shown a change in CQD shape and optical characteristics as the CQDs grow in size. This work aims at understanding the effect of size, shape, and composition on the sensitive optical behavior in this particular class of CQDs using density functional theory (DFT) code Octopus. Resonant optical transitions and energy relaxation pathways are elucidated, with emphasis towards activity in the 3-5 µm mid-wave infrared (MWIR) spectrum. Optoelectronic results are presented for the bulk material, isolated slabs representative of specific crystalline facets, and for the octahedral through cubic morphologies that evolve with CQD growth.

Ab Initio Studies of the Optical Properties of Lead Chalcogenide Nanocrystals

Abstract Body: There is continued interest in the combination of Si and Ge in unique geometries to further improve the performance of transistors [1]. A novel diffusion mechanism of Ge along an oxidizing Si/SiO2 interface was reported, which resulted in the formation of strained Si nanowires [2]. By taking advantage of this diffusion mechanism, there is the potential of forming site-controlled nanowires, quantum dots, and arbitrary nanoshapes. However, there is limited understanding of the diffusion mechanism that results in the formation of these nanostructures. To investigate this mechanism, alternating 15nm thick layers of Si and Si0.7Ge0.3were grown in a superlattice and patterned into fins via electron beam lithography. The final pattern contained multi-layered Si/SiGe fins with widths varying from 70 to 280nm with a 100nm layer of Si substrate exposed at the base. This extended Si surface enabled the observation Ge’s diffusion down the fin’s sidewall during high temperature oxidation. Ge’s lateral diffusion was observed over a range of times at temperatures between 800 and 950°C. Cross-sectional TEM samples were prepared after each anneal and the diffusion length was measured through analysis of HAADF-STEM images, utilizing z-contrast. The diffusion length was measured after four different oxidation times at each temperature and the diffusivity was extracted. An Arrhenius plot of the data indicated the activation energy of Ge diffusion down the oxidizing interface is 2.50 +/- 0.3 eV. This is approximately half of activation enthalpy for Ge’s movement into bulk Si or through the oxide [3], [4]. We propose that the Ge may be diffusing through the suboxide along the Si/SiO2 interface and is subsequently reinserted back into the Si during oxidation. This is further supported by the linear correlation between the thickness of the thermally grown oxide and the distance the Ge diffuses down the Si sidewall. The difference in the activation energy compared to bulk interdiffusion helps to explain why the nanowires and quantum dots are formed within these materials during oxidation. In addition to dry oxidation anneals, anneals in a wet oxidizing ambient (steam) will be discussed. A model for the diffusion using the Florida object-oriented process and device simulator (FLOOXS) was developed. This model was used to assist in the extraction of diffusivities and is being further developed to help predict possible structures using this phenomenon. Preliminary results using the process simulator’s model will be compared to the experimentally observed evolution of the nanostructures. This work is supported by Sandia National Laboratories, which is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. [1] T. David et al., “New strategies for producing defect free SiGe strained nanolayers,” Sci. Rep., vol. 8, no. 1, pp. 1–10, Feb. 2018, doi: 10.1038/s41598-018-21299-9. [2] W. M. Brewer, Y. Xin, C. Hatem, D. Diercks, V. Q. Truong, and K. S. Jones, “Lateral Ge Diffusion During Oxidation of Si/SiGe Fins,” Nano Lett., vol. 17, no. 4, pp. 2159–2164, Apr. 2017, doi: 10.1021/acs.nanolett.6b04407. [3] R. Kube et al., “Composition dependence of Si and Ge diffusion in relaxed Si1−xGex alloys,” J. Appl. Phys., vol. 107, no. 7, p. 073520, Apr. 2010, doi: 10.1063/1.3380853. [4] Y. Dong, Y. Lin, S. Li, S. McCoy, and G. Xia, “A unified interdiffusivity model and model verification for tensile and relaxed SiGe interdiffusion over the full germanium content range,” J. Appl. Phys., vol. 111, no. 4, p. 044909, Feb. 2012, doi: 10.1063/1.3687923.

Rapid Ge Diffusion Along Si/SiO2 Interfaces During High Temperature Oxidation for Quantum-Scale Structures

Abstract Body: With the discovery of topological insulators, a wide list of potential applications for their unique spin-momentum locked surface states have been proposed. Among these possibilities, it has been posited that electrons occupying these surface states might be useful as information carriers, since they cannot be scattered by a non-magnetic perturbation. A quantum dot-like system with discrete states that features these properties is of interest. To explore the potential of producing such a device, we first must learn more about how to make topological insulator quantum dots, and then study whether the topologically-protected surface states exhibit discretization while maintaining their unique properties. This study focuses on the growth and characterization of self-assembled, selenized bismuth particles on GaAs via droplet epitaxy in a molecular beam epitaxy system. In droplet epitaxy, we first deposit a small amount of bismuth which has a high surface tension and does not wet the substrate, resulting in the formation of bismuth clusters. These clusters are then exposed to a selenium flux, forming Bi2Se3 nanoparticles. For this study, growth time and substrate temperature during deposition were varied, and final particle density, average height and area were measured via a combination of atomic force microscopy and scanning electron microscopy. Within the substrate temperature (250-285 οC) and bismuth deposition time (20-100 seconds) ranges, we observed variations in average height, area, particle density, and polydispersity. Within the growth window, particles are generally 5-15 nm tall, and 500-3000 nm2 in area (equivalent circular radius ~12-30 nm). When the amount of bismuth initially deposited increases, the nanoparticle density also increases, though this effect becomes less significant at higher substrate temperatures. Above 275 οC, the average particle height and area settle around 10 nm and 500-1000 nm2 (equiv. circular radius ~12-17 nm). Polydispersity in particle height and area tends to drop relatively significantly above 275 οC as well. There also appears to be some correlation between particle height and area when changing growth conditions: in general, if particles are becoming taller on average, they are also becoming larger in radius. We demonstrated that adding a long anneal step between the bismuth deposition and selenium exposure steps resulted in smaller, shorter, and more numerous particles. These observations support our over-arching theory that the nanoparticle size is determined by bismuth adatom mobility and rate of bismuth removal from an existing nanoparticle rather than nanoparticle mobility. We have demonstrated the ability to affect the properties of particle height, area, density, and polydispersity by changing the initial bismuth deposition time and substrate temperature. These results should prove useful to those interested in creating topological insulator devices through epitaxial growth. Acknowledgments: National Science Foundation, Division of Materials Research under Award No. 1838504 and the Brookhaven National Laboratory/University of Delaware Seed Program under Award No. 20A00145. We also acknowledge the use of the Materials Growth Facility (MGF) at the University of Delaware, which is partially supported by the National Science Foundation Major Research Instrumentation under Grant No. 1828141 and is a member of the Delaware Institute for Materials Research (DIMR).

Growth of Topological Insulator Bi2Se3 Particles on GaAs via Droplet Epitaxy

Abstract Body: Alloy quantum dots (aQD) represent a new class of low-dimensional materials whose electron confinement is determined not exclusively by size, but also by composition field. While homogeneous quantum dots have been more widely explored for applications in photonics, optoelectronics, and photovoltaics, aQDs remain less investigated. The differences in both size and composition field across aQDs lead to differences in material properties. These variations in material properties present compelling potential applications, specifically in the field of energy generation and photovoltaics. This is primarily because of the complexity in characterizing nonuniform fields and establishing the correlation between inhomogeneity and confinement. In order to effectively analyze the varied electron confinement of these materials, we employ a combination of density functional theory based quantum simulations, and finite element analysis based k.p calculations to determine the electron energy levels for a number of uniform and nonuniform physical variables. As a part of the analysis, composition maps were constructed for each of the different aQD shapes, namely, the steep cone, normal cone, hut, and dome. Each of these compositions maps was fitted with an analytical function that was then used to analyze the electronic properties of the aQDs. Results show that a generic expression for all four of the aQDs exists such that modification of this equation allows each of the disparate aQDs to be modeled. The steep cone shaped aQD with a 15° sidewall angle corresponds to a composition map where spatial decay occurs from the tip of the cone to the bottom. The normal cone shaped aQD exhibits a similar behaviour, but the sidewall angle is instead 30°, and the fitting function is modified to a product of a quadratic and a linear function. For the hut shaped aQD, the composition map takes the shape of a “hut” where decay occurs from the top edges towards the bottom edges in a non-linear fashion. For the final shape, the dome, spatial decay emanates from the top tip of the dome. The dome is fitted by two sidewalls per side that have angles of 11° and 30° respectively. For each of the quantum dots above, composition maps obtained are independent of size to determine the singular effects of the compositional variation. Our results suggest that inhomogeneity in composition field has a consequential role in controlling the confinement energy levels. Thus, the shape of aQDs can play a critical role in tailoring the effective electronic behavior. Furthermore, a larger alloy quantum dot, can behave electronically like a smaller quantum dot, if the composition field is inhomogeneous. By engineering quantum dots with certain composition maps, the photon absorption properties of the respective quantum dot can thus be tailored substantially. In particular, the most apparent application of this is to solar cells and spintronics; engineering the properties of the quantum dot allows us to increase the absorption efficiency of the photovoltaic cell, which directly leads to greater potential for green energy generation.

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